Pre-Alpine ophiolites in the basement of Southern Alps: the presence of a bimodal association (LAG - Leptyno-Amphibolitic Group) in the Serie dei Laghi (N-Italy, Ticino-CH)

In several portions of the Hercynian belt, a peculiar bimodal lithological association is found. Its name, Leptyno-Amphibolitic Group, reflects its being made of alternating cm-thick, fine-grained leucocratic and melanocratic layers of amphibolite facies metamorphic rocks. In the «Strona Ceneri Border Zone» (SCBZ), a part of the Southalpine basement, the banded amphibolites, contain lenses of metagabbro, retrogressed eclogites and ultramafites. To discriminate among the hypotheses on their origin proposed in the literature, we obtained major and trace elements concentrations and Sr-Nd (Pb) isotopic data. All data concordantly indicate that the SCBZ banded amphibolites derive from arc/back arc bimodal volcanics whose emplacement occurred in an Early Paleozoic convergent zone. The leptynites show geochemical characters,i.e. high HREE and (Tb/Lu)N=1.11 (average), and isotopic composition (eNd from +2.66 to +3.49;87Sr/86Sr466=0.7034−0.7053) that are clearly different from those of the Strona Ceneri metasiliciclastic rocks like the Gneiss Minuti which show an average (Tb/Lu)N=1.52 and eNd466=−6.69. Their isotopic composition is also completely different from that of the Ordovician intrusives of Serie dei Laghi (eNd466 0.7078). The fine-grained amphibolitic layers are clearly derived from basaltic tuffites and have eNd466 from +3.21 to +7.73. Their geochemistry is compatible with a derivation from back-arc tholeiites. The SCBZ, including the banded amphibolites, may well represent the reworked products of an early Hercynian suture, in which ophiolitic remnants that underwent HP metamorphism in a subduction zone, have been incorporated in turbiditic sediments derived from back-arc bimodal volcanites prior to the intrusion of the Ordovician granites that cut across all the rocks belonging to the Serie dei Laghi